In the semiconductor industry, there is an incentive to reduce the size of each semiconductor device on a chip. A smaller device can allow an increased density of devices on a chip and can help semiconductor manufacturers reduce cost and improve performance of a chip. When the size of a DRAM device or cell is reduced, more DRAM cells can fit on a chip of a certain size, resulting in increased memory storage capability of the chip.
The size of a DRAM device may be reduced by designing the DRAM device or cell in three-dimensions. This may reduce the semiconductor surface area used by each DRAM cell. For example, using trench capacitors in a DRAM cell may result in a reduced semiconductor surface area used by the DRAM cell. FIG. 1 shows a cross section of a buried plate trench DRAM cell 100 as described by Donald M. Kenney in U.S. Pat. No. 5,348,905.
The DRAM cell 100 in FIG. 1 is formed in a p-type semiconductor substrate 10. A p-type well 12 is formed in the upper surface of the substrate 10. An n-channel transfer device 14 is formed within the p-type well 12. The transfer device 14 couples data between a bit-line (not shown) that is connected to the bit-line diffused n-type region 18 and the diffused n-type storage node region 20. The gate electrode 16 is connected to a word line (not shown) and controls the transfer of data through the channel 34 of the transfer device 14.
A storage capacitor is formed in a deep trench 22 adjacent diffused n-type storage node region 20. The storage capacitor includes a signal storage node 24 in the deep trench 22 which is isolated from substrate 10 by a thin dielectric layer (not shown). The diffused n-type storage node region 20 and the signal storage node 24 in the deep trench 22 are connected by a conductive strap 26. An insulating collar 28 decreases leakage of stored charge between diffused n-type storage node region 20 and buried n-type layer 32. The surface region of substrate 10 occupied by the DRAM cell 100 is defined by local surface isolation 30.
The buried n-type layer 32 acts as the reference voltage node for the trench capacitor and forms electrical and physical isolation between the p-type well 12 and the substrate 10. As known to those skilled in the art, reference voltages V.sub.bn, V.sub.bp, and V.sub.sub may be used to bias the device 100.
FIGS. 2A-2D illustrate a three-sided gated DRAM cell in a diagonal bit-line configuration. K. Shibahara et al., 1GDRAM Cell With Diagonal Bit-Line (DBT) Configuration and Edge Operation MOS(EOS) FET, IEDM Technical Digest, 639-42 (1994). As shown in FIGS. 2A and 2B, the device 200 is formed in a substrate 202. The device 200 has a channel 206 coated with a gate oxide 208 and a gate conductor 204. The storage node of the device 200 is a stacked capacitor. Id. at 640.
FIG. 2B shows the device 200 in operation where an inversion layer 210 is formed at a corner 212 of the channel 206. FIG. 2C shows the layout of a device 200, defined by a boundary 216, that occupies a 6F.sup.2 (3F.times.2F) size area of the substrate 202, where F is the minimum lithographic dimension. As shown in FIG. 2C, the device 200 does not have any sub-lithographic dimensions.
Recessed LOCOS (local isolation of silicon) isolation regions are used to isolate the device 200 rather than using shallow trench isolation (STI) regions. Id. at 640 (see first paragraph). As shown in FIG. 2D, the LOCOS isolation regions form a birds beak 214 which may result in enough mechanical stress to cause dislocations of the silicon lattice and cause silicon defects.
FIG. 3 illustrates a MOSFET (metal oxide semiconductor field effect transistor) device 300 gated on three sides. K. Hieda et al., Effects of a New Trench-Isolated Transistor Using Sidewall Gates, IEEE Transactions on Electron Devices, Vol. 36(9), 1615-19 (1989). The device 300 formed in a substrate 302 includes a drain 304, a source 306, and a three-sided gate 308. Isolation regions 310 are formed on both sides of the device 300. Each isolation region 310 is formed in a trench coated with an oxide layer 312 and then filled with polysilicon 314.
By using a three-sided gate 308, sidewall conduction of the channel 316 may be achieved. K. Hieda et al. teach that the above MOSFET device 300 is compatible with isolation merged DRAM and folded capacitor cells (see page 1616, column 2, first full paragraph) also known as stacked-in-trench cells. The device 300 is formed without a channel tailor implant on the sidewalls of the channel 316 (see page 1616, column 1, last paragraph).
To overcome the shortcomings of conventional DRAM devices, a new DRAM device is provided. An object of the present invention is to provide an improved DRAM device that has a reduced leakage current. A related object is to provide a process of manufacturing such a DRAM device. Another object is to provide a DRAM device that occupies a reduced semiconductor area. Another object is to provide a DRAM device having a sub-lithographic channel width on a substantially planar substrate surface. It is still another object to provide a DRAM device having a transfer device with a channel controlled from three sides. Yet another object is to provide a DRAM device having improved charge characteristics.